Device and method for preparing a treatment liquid by filtration

ABSTRACT

A device for treatment of blood by extracorporeal circulation comprises a dialysis liquid circuit having a supply tube (9a, 9b) on which is arranged a filter (21) having a first chamber and a second chamber (22, 23) which are separated by a filtering membrane (24), the supply tube having a first portion (9a) connecting a dialysis liquid source (10) to an inlet of the first chamber (22), and a second portion (9b) having an end connected to an outlet of the second chamber (23) of the filter (21) and another end which can be connected to an inlet of a compartment (3) of an exchanger (1) with semipermeable membrane (4). A feedback tube (25) connects the inlet of the first chamber (22) of the filter (21) to the outlet of the first chamber (22). A flushing pump (26) is arranged on the feedback tube (25) for circulating liquid in the first chamber (22) of the filter (21) and causing cleaning of the membrane (24) by tangential flushing.

This is a division of application Ser. No. 08/462,435, filed Jun. 5,1995, now U.S. Pat. No. 5,702,597.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject of the present invention is a device for treatment of bloodby extracorporeal circulation, designed to produce a sterile and pyrogenfree treatment liquid.

2. Description of the Related Art

Patients suffering from renal insufficiency may undergo varioustreatments involving extracorporeal circulation of blood, haemodialysis,haemofiltration and haemodiafiltration.

Haemodialysis consists in circulating the blood of the patient in afirst compartment of a dialyser, and a dialysis liquid in a secondcompartment of the dialyser, the two compartments being separated by asemipermeable membrane allowing diffusive transfer of solutes throughthe membrane, from the compartment in which the concentration of aparticular solute is the highest to the compartment in which theconcentration of this solute is the lowest.

Haemofiltration consists in extracting a fraction of plasmatic water orfiltrate from the blood of the patient by means of a haemofilter and insimultaneously infusing into the patient a substitution liquid in orderto compensate, in general only partially, for the quantity of filtrateextracted.

Haemodiafiltration is a combination of the two aforementionedtreatments.

The dialysis liquid and the substitution liquid are liquids which havesubstantially the same composition. They are isotonic with the blood ofwhich they contain the main electrolytes.

In conventional dialysis machines, the dialysis liquid is prepared bymixing water and concentrated solutions or pulverulent salts comprisingthe main electrolytes of blood. This dialysis liquid is neither sterilenor pyrogen free, that is to say that it may contain livingmicroorganisms (bacteria) as well as components called pyrogens, theintroduction of which into the body may produce undesirable effects,such as fever, shivering, nausea or anaphylactoid reactions.

Although the membrane of the dialyser isolates the blood from thedialysis liquid and, during treatment, a positive transmembrane pressureis set up between the blood compartment and the dialysis-liquidcompartment of the dialyser in order to prevent the passage of dialysisliquid into the blood, not all contamination of the blood by bacteriaand pyrogenic components contained in the dialysis liquid is completelyexcluded, especially in the event of breakage of the filter or in theevent of accidental reversal of the direction of the transmembranepressure when a dialyser with high hydraulic permeability is used.

Repeated interest in the use of sterile and pyrogen free dialysis liquidhas moreover been expressed. It has, in particular, been proposed tofilter the dialysis liquid in a filter having a first chamber and asecond chamber which are separated by a membrane having a high hydraulicpermeability, the first chamber having an inlet for introducing theliquid to be filtered and an outlet for discharging the substancestrapped by the membrane together with a fraction of the liquidintroduced into the filter (see "Investigation of the Permeability ofHighly Permeable Polysulfone Membranes for Pyrogens" in Contr. Nephrol.,vol. 46, pp. 174-183, Karger, Basel 1985).

European Patent 0,270,794 describes a dialysis machine whose dialysisliquid circuit comprises:

a supply tube on which is arranged a filter having a first chamber and asecond chamber which are separated by a filtering membrane, the supplytube having a first portion connecting a dialysis liquid source to aninlet of the first chamber, and a second portion having an end connectedto an outlet of the second chamber of the filter and another end whichcan be connected to an inlet of a first compartment of a dialyser;

a discharge tube having an end which can be connected to an outlet ofthe first compartment of the dialyser; and

a purge tube on which a valve is arranged connecting an outlet of thefirst chamber of the filter to the discharge tube.

During operation, the valve arranged on the purge tube is opened atregular time intervals in order to purge the first chamber of the filterof the microorganisms and pyrogenic components trapped by the membrane.

This machine has several drawbacks. In particular, it is clear that thevalve of the purge tube cannot be opened frequently because of theinterruption to the treatment which results therefrom. (As a matter offact, because of the significant head loss which the membrane of thefilter constitutes, when the valve is opened, any dialysis liquid comingfrom the dialysis liquid source flows through the purge tube and thedialyser is no longer supplied with fresh dialysis fluid). Furthermore,between two successive openings of the valve, there is a time lag duringwhich the undesirable substances accumulate in the first chamber of thefilter where they tend to be entrained by convection towards themembrane and to clog it. The result of this is, in the event of breakageof the membrane of the filter, that these accumulated substances aresent into the dialyser and the dialysis circuit portion located upstreamand downstream thereof, which these substances contaminate. Also, thetangential flushing of the membrane which results from intermittentopening of the valve cannot be sufficiently long to detach from themembrane all the substances which have adhered thereto.

SUMMARY OF THE INVENTION

One object of the invention is to produce a dialysis/haemofiltrationmachine capable of producing a treatment liquid (dialysis liquid,substitution liquid) which is made sterile and pyrogen free byfiltration, in which the cleaning of the filter used causes little or nointerruption to the treatment being carried out and makes it possible tooptimize the working life of the filter.

In order to achieve this object, a device for treatment of blood byextracorporeal circulation is provided, according to the invention,comprising a dialysis liquid circuit having:

a supply tube on which is arranged a filter having a first chamber and asecond chamber which are separated by a filtering membrane, the supplytube having a first portion connecting a dialysis liquid source to aninlet of the first chamber, and a second portion having an end connectedto an outlet of the second chamber of the filter and another end whichcan be connected to an inlet of a compartment of an exchanger withsemipermeable membrane;

a discharge tube having an end which can be connected to an outlet ofthe compartment of the exchanger; and

a purge tube on which a flow-control element (for example a valve) isarranged and which is connected to an outlet of the first chamber of thefilter,

characterized in that it comprises a feedback tube connecting the inletof the first chamber of the filter to the outlet of the first chamber,and on which is arranged a flushing pump for circulating liquid in thefirst chamber of the filter and causing cleaning of the membrane bytangential flushing.

By virtue of this arrangement, the cleaning of the membrane bytangential flushing is optimal, on the one hand, because this flushingis continuous and, on the other hand, because, being independent of thecirculation rate of the dialysis liquid in the dialysis liquid circuit,it can be adjusted (by setting the flushing pump) so as to be asefficient as possible. The working life of the filter is directly linkedwith the efficiency of the cleaning to which it is subjected.

According to one characteristic of the invention, the device comprisesmeans for detecting clogging of the membrane of the filter and controlmeans for controlling the flow-control element of the discharge tube asa function of a predetermined clogging threshold of the membrane so asto cause purging of the first chamber of the filter.

By virtue of this arrangement, the frequency of the purges of the firstchamber of the filter only depends on the bacteria and pyrogens contentof the dialysis liquid to be filtrated so that, if the water used forpreparing the dialysis liquid is very clean, the valve of the purge tubeis not open or only once or twice per treatment session.

According to another characteristic of the invention, the devicecomprises means for calculating the frequency of the purges andcomparing the calculated frequency with a reference frequency; thecontrol means are then designed to increase the flow rate of theflushing pump when the calculated frequency of the purges reaches thereference frequency.

In this way, the intensity of the cleaning of the filter can be adjustedas a function of the rate at which it clogs up, that is to say also as afunction of the degree of purity of the dialysis liquid coming from thedialysis liquid source.

According to yet another characteristic of the invention, the deviceincludes means for storing in memory an initial transmembrane pressurewhen the filter is set in operation for the first time and means forcomparing the initial transmembrane pressure with the transmembranepressure measured. The device may furthermore include alarm means foremitting an alarm when the transmembrane pressure measured deviates by apredetermined quantity from the initial transmembrane pressure.

It has been observed that, with equal dialysis liquid flow rate in thedialysis liquid circuit, the transmembrane pressure in the filterincreases over time, so that the comparison between the transmembranepressure when the filter is set in operation and the transmembranepressure measured at a given instant in the use of the filter gives anaccurate idea of the ageing of the filter. It is therefore possible todefine an optimal individual working life for each filter, correspondingto a relative increase in the transmembrane pressure.

According to yet another characteristic of the invention, the deviceincludes means, such as a flow meter, for monitoring a quantity ofdialysis liquid, which means are arranged on the supply tube, and thefilter is placed on the supply tube between the dialysis liquid sourceand the means for monitoring a quantity of dialysis liquid.

This arrangement has several advantages: first, a filtered liquid passesthrough the monitoring means, which is entirely suitable when thesemeans consist of a flow meter, and in particular a turbine or gear flowmeter whose operation may be seriously disturbed by a solid impurity andis generally degraded by progressive fouling. Furthermore, suchmonitoring means may form part of a volumetric ultrafiltration controlsystem by means of which, in a portion of the dialysis liquid circuitcomprising the exchanger, the quantity of dialysis liquid leaving thiscircuit portion is kept equal to the quantity of dialysis liquid whichenters it. Second means for monitoring the quantity of dialysis liquidare then arranged on the discharge tube of the dialysis liquid circuit.By connecting the filter to the dialysis liquid circuit outside theportion of the dialysis liquid circuit in which a constant volume ofliquid circulates, the necessity of having to connect the purge tube tothe discharge tube of the dialysis circuit upstream of the secondmonitoring means is avoided, that is to say also of having to introducetherein the impurities trapped by the filter as is the case in thedevice described in European Patent 0,270,794 mentioned hereinabove. Yetanother advantage of this arrangement is that the purge tube can bedirectly connected to the drain by an outlet channel of the dialysismachine which is independent of the outlet channel by which thespent-liquid discharge tube is connected to the drain. In this way, theportion of the dialysis liquid circuit which is located upstream of thefilter is completely isolated by the filter from the dialysis liquidcircuit portion located downstream of the filter. Contamination of thedischarge tube, both downstream and upstream (back-contamination) of thejunction of the purge tube with the discharge tube, which occurs on thedevice described in the prior art, is therefore completely avoided.

A further subject of the invention is a method for cleaning a filterhaving two chambers separated by a filtering membrane, a first chamberbeing connected to a treatment liquid source, a second chamber having anoutlet for the filtered treatment liquid, characterized in that itconsists in continuously recirculating treatment liquid into the firstchamber in order to cause tangential flushing of the membrane of thefilter and to prevent clogging of the membrane by the substances stoppedby the membrane.

According to one characteristic of the invention, the method furthermorecomprises the steps of:

measuring a clogging level of the filter;

comparing the clogging level measured with a predetermined cloggingthreshold; and

purging the first chamber of the filter when the clogging level measuredreaches the predetermined clogging level.

According to another characteristic of the invention, the methodfurthermore consists in comparing the actual frequency of the purges ofthe first chamber of the filter with a reference frequency and inincreasing the rate of recirculation into the first chamber of thefilter when the actual frequency reaches the reference frequency.

The method may be particularly adapted to a filter arranged in avolumetric ultrafiltration control system of ahaemodialysis/haemofiltration machine, the ultrafiltration controlsystem comprising two elements for monitoring a quantity of liquid, suchas two flow meters, arranged on a dialysis liquid circuit respectivelyupstream and downstream of a haemodialyser, the two flow meters beingdirectly connected in series in repetitive fashion by a branch line tothe haemodialyser in order to be subjected to calibration. In thisarrangement, according to yet another characteristic of the invention,purging of the first chamber of the filter is ordered during calibrationof the flow meters following the instant when the clogging levelmeasured reaches the predetermined clogging threshold.

A further subject of the invention is a method for determining theageing of a filter having two chambers separated by a filteringmembrane, a first chamber being connected to a treatment liquid sourceand a second chamber having an outlet for the filtered treatment liquid,characterized in that it comprises the steps of:

measuring and storing in memory, for a given treatment liquid flow rate,a reference transmembrane pressure in the filter the first time thefilter is set in operation;

measuring, on each subsequent use of the filter, the transmembranepressure at the given treatment liquid flow rate;

comparing the transmembrane pressure measured with the referencetransmembrane pressure.

Other characteristics and advantages of the invention will emerge onreading the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the attached drawings, in which:

FIG. 1 is a diagram of a dialysis device comprising a dialysis-liquidfiltration system according to the invention; and

FIG. 2 is a diagram of a haemodiafiltration device comprising atreatment-liquid filtration system according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device for treatment of blood by extracorporeal circulationrepresented in FIG. 1 is suitable for carrying out a dialysis treatment.This device comprises a haemodialyser 1 having two compartments 2, 3which are separated by a semipermeable membrane 4, a first compartment 2being connected to an extracorporeal blood circulation circuit, a secondcompartment 3 being connected to a dialysis liquid circulation circuit.In conventional manner, all the components of the device which will bedescribed hereinbelow, with the exception of the blood circulationcircuit and the haemodialyser, are arranged within a so-called dialysismachine.

The blood circulation circuit comprises a withdrawal tube 5 connected toan inlet of the first compartment 2, on which tube a pump 6 is arranged,and a delivery tube 7 connected to an outlet of the first compartment 2,on which tube a bubble trap 8 is mounted.

The dialysis liquid circuit comprises a supply tube (9a, 9b) for freshdialysis liquid, connecting a dialysis liquid source 10 to an inlet ofthe second compartment 3 of the haemodialyser 1, and a discharge tube 11for spent liquid, connecting an outlet of the second compartment 3 ofthe haemodialyser 1 to the drain.

The dialysis liquid source is, for example, a dialysis liquid generator10 using which water is heated, degassed then mixed, in determinedproportions, with concentrated solutions containing the mainelectrolytes of blood. The dialysis liquid produced is neither sterilenor pyrogen free.

The dialysis liquid circuit comprises dialysis liquid circulation meansconsisting of a first pump 12 arranged on the supply tube (9a, 9b) and asecond pump 13 arranged on the discharge tube 11.

The device also comprises a volumetric ultrafiltration control systemcomprising a first means for monitoring a quantity of dialysis liquid,such as a first flow meter 14, arranged on the supply tube (9a, 9b,) anda second means for monitoring a quantity of dialysis liquid, such as asecond flow meter 15, arranged on the discharge tube 11 downstream ofthe second circulation pump 13. The measurements taken by the two flowmeters are compared by a calculation and control unit 16 which drivesthe second dialysis liquid circulation pump 13 so that the flow ratesmeasured by the two flow meters are identical. The ultrafiltrationcontrol system furthermore comprises an ultrafiltration pump 17connected to the discharge tube upstream of the second pump 13. Byvirtue of the above-described slaving of the second pump 13, thequantity of liquid extracted from the dialysis liquid circuit by theultrafiltration pump 17 corresponds exactly to the quantity of plasmaticwater which passes from the blood into the dialysis liquid byultrafiltration through the membrane 4 under the effect of the relativepressure reduction created in the dialysis liquid circuit by theultrafiltration pump 17. A branch tube 18 connects the supply tube (9a,9b) to the discharge tube 11, to which tubes it is connected, via twothree-way valves 19, 20, respectively downstream of the first flow meter14 and upstream of the second flow meter 15. This branch line 18 makesit possible to arrange the flow meters 14, 15 directly in series forprogrammed calibration at regular intervals.

The device furthermore comprises a filter 21 for filtering the dialysisliquid produced by the dialysis liquid generator 10. The filter 21 has afirst chamber and a second chamber 22, 23 which are separated by afiltering membrane 24, the first chamber 22 having an inlet connected toa first portion 9a of the supply tube and the second chamber 23 havingan outlet connected to a second portion 9b of the supply tube, on whicha valve 31 is arranged.

According to the invention, a feedback tube 25, on which a flushing pump26 is arranged, connects an outlet of the first chamber 22 of the filter21 to the inlet of this first chamber. A tube 27 for purging the firstchamber 22 of the filter 21, on which tube a flow-control element suchas a valve 28 is arranged, is connected to the feedback tube 25 betweenthe outlet of the first chamber 22 of the filter 21 and the pump 26. Thepurge tube 27 is connected to the drain by an outlet of the dialysismachine which is separate from the outlet which forms the end of thedischarge tube 11 for the spent liquid.

Two pressure sensors 29, 30 are arranged, respectively, on the secondportion 9b of the supply tube and on the feedback tube 25, at the outletof the first and second chambers 22, 23 of the filter 21, in order tomeasure the pressure in these chambers. The information delivered by thepressure sensors 29, 30 is supplied to the control and calculation unit16 which can calculate the transmembrane pressure in the filter 21 andcontrol the operation of the device as a function of the measured andcalculated values of the pressures in the filter 21, as will beexplained hereinbelow.

When the steps preliminary to the treatment have been completed, that isto say the initial rinsing and filling of the dialysis liquid circuit9a, 9b, 11, of the haemodialyser 1 and of the blood circuit 5, 7, 8 andthe connection of the blood circuit to the vascular circuit of thepatient, the dialysis liquid produced by the dialysis liquid generator10 is set in circulation in the dialysis liquid circuit by means of thepumps 12 and 13, and the blood of the patient is set in circulation inthe blood circuit by means of the pump 6 (the valve 31 is then openedand the valves 19, 20 are arranged so as to allow circulation in thesupply tube 9a, 9b and in the discharge tube 11).

Furthermore, the valve 28 of the purge tube 27 is closed and theflushing pump 26 rotates at a predetermined speed so that unfilteredliquid coming from the generator circulates continuously in the firstchamber 22 of the filter 21 and flushes the membrane 24, which has theeffect of opposing its clogging by keeping in suspension the impuritiesstopped by the membrane of the filter.

In a conventional manner, the flow meters 14 and 15 are calibrated atregular time intervals. During these successive calibrations, the valves19 and 20 are arranged so that the dialysis liquid circulates in thebranch tube 18, and the first circulation pump 12 and theultrafiltration pump 17 rotate at their set-point value, whilst thesecond circulation pump 13 is stopped.

According to the invention, the purging of the first chamber 22 of thefilter 21, intended to remove the impurities stopped by the membrane 24,is ordered when a predetermined clogging level of the membrane isdetected. More precisely, the control unit 16 compares, eithercontinuously or at regular time intervals, the instantaneous value ofthe transmembrane pressure in the filter 21, calculated on the basis ofthe information delivered by the pressure sensors 29, 30, with areference value calculated at the start of the session. When theinstantaneous value exceeds the reference value by a predeterminedquantity, a purge of the first chamber 22 of the filter 21 is orderedduring the subsequent calibration phase of the flow meters 14, 15. Thevalve 31 is then closed and the purge valve 28 is opened for the timenecessary for discharging, via the purge tube 27, the liquid containedin the first chamber 22 of the filter 21 and in the feedback tube 25.

According to the invention, the frequency of the purges is calculated bythe calculation unit and is compared with a reference frequency. If thefrequency calculated reaches or exceeds the reference frequency, thespeed of rotation of the pump 26 is increased so that the efficiency ofthe cleaning of the membrane by tangential flushing is enhanced.

According to the invention, the ageing of the filter 21 is measured bystarting with the observation that, during time and despite thecontinuous cleaning to which the membrane 24 is subjected, thetransmembrane pressure in the filter rises, for equal dialysis liquidflow rates. The control and calculation unit 16 therefore compares, atregular time intervals, a reference transmembrane pressure whichcorresponds, for example, to the transmembrane pressure of the filtercalculated the first time it is set in operation for a fixed dialysisliquid flow rate, with the instantaneous transmembrane pressure measuredwith the same flow rate, and when the latter deviates from the former bya predetermined quantity, it emits an alarm or a message on a displayunit (not shown) in order to signal to the user that the filter 21should be changed.

FIG. 2 represents a haemodiafiltration device which differs from thehaemodialysis device described hereinabove by the followingcharacteristics (the components of these two devices which fulfill thesame functions have been denoted by the same reference numbers).

In accordance with its purpose, this device includes means for infusinginto the patient a substitution liquid intended to compensate, generallyin part, for the quantity of plasmatic water withdrawn from the vascularcircuit of the patient by ultrafiltration in the high-permeabilityhaemodialyser 1. The infusion means comprise a substitution liquid tube32 having a first portion connecting the dialysis-liquid supply tube 9a,9b to an inlet of a first chamber 34 of a second filter 33, and a secondportion connecting an outlet of a second chamber 35, separated from thefirst chamber 34 by a filtering membrane 36, to the bubble trap 8 of theextracorporeal blood circulation circuit. A substitution liquidcirculation pump 37 is arranged on the second portion of thesubstitution liquid tube 32. The substitution liquid tube is connectedto the supply tube 9a, 9b between the element 14 for measuring aquantity of dialysis liquid (for example flow meter) and the firstdialysis liquid circulation pump 12.

In this device, the first filter 21 is not connected to thedialysis-liquid supply tube 9a, 9b inside the volumetric ultrafiltrationcontrol system (flow meters 14, 15, portion of the dialysis liquidcircuit extending between the flow meters and the ultrafiltration pump17) but outside this system, upstream of the first flow meter 14. Asmentioned hereinabove, by virtue of this arrangement, the liquidcirculating in the first flow meter 14 is a filtered liquid, whichprevents fouling and, should the need arise, accidental blocking of thissensitive measurement element.

Another difference with the device represented in FIG. 1 is that theflushing pump 26 is arranged immediately upstream of the first chamber22 of the first filter 21, on a tube portion common to the supply tube9a and to the feedback tube 25. In operation, the flushing pump 26 isset to a flow rate greater than the sum of the flow rates of thecirculation pump 12 and of the substitution liquid pump 37, so that adetermined recirculation rate is set up in the feedback tube 25. Arestricting element 38, which may optionally be adjustable, is arrangedon the feedback tube 25, by virtue of which it is possible to obtain asufficient pressure in the dialysis liquid circuit upstream of thecirculation pump 12 for the pressure in the second chamber 35 of thesecond filter 33, as measured by a pressure sensor 40, to be alwayspositive or at least zero, whatever the flow rate of the substitutionliquid pump 37. The restriction element 38 may consist of a calibratedportion of the feedback tube 25.

The operation of this second device is as follows, in so far as itdiffers from that of the device represented in FIG. 1.

With the various pumps of the installation rotating at the ratesprescribed by the operator or programmed by default or calculated,either at the start or during the session, the calculation and controlunit 16 compares, either continuously or at regular time intervals, thepressure measured by means of the sensor 29 in the second chamber 23 ofthe filter 21 with a set-point pressure and it drives the flushing pump26 so that the instantaneous pressure tends towards the set-pointpressure. The set-point pressure is chosen so that the pressure in thesecond chamber 35 of the second filter 33, as measured by the pressuresensor 40, is always positive or at least zero, taking into account thesubstitution liquid flow rate imposed on the pump 37.

In order to measure the degree of clogging of the membrane of the filter21, two methods are possible. Either, as described hereinabove, thetransmembrane pressure in the filter 21 is calculated by virtue of thedata supplied by the pressure sensors 29, 30 and is compared with areference transmembrane pressure stored in memory (for example thetransmembrane pressure measured at the start of the session). Or thespeed of rotation of the flushing pump 26 is compared with a referencespeed (for example the speed of rotation measured at the start of thesession and stored in memory in the control unit 16). Since the pressureis kept constant in the second chamber 23 of the filter 21 by theregulating of the speed of rotation of the flushing pump 26, if themembrane 24 becomes clogged, the speed of rotation of the pump will beincreased commensurately. When the measured value of the parameter inquestion (transmembrane pressure or speed of rotation of the flushingpump) reaches the corresponding reference value, the control unit 16commands opening of the purge valve 28 for the time necessary todischarge the liquid contained in the first chamber 23 of the filter 21and in the feedback tube 25.

The invention is not limited to the specific embodiments which have justbeen described, and it may comprise variants. In particular, althoughthis variant may be considered to be less advantageous, the purge tube27 of the device in FIG. 1 might be connected to the discharge tube 11for spent liquid inside the volumetric ultrafiltration control system,that is to say upstream of the second means 15 for measuring a quantityof spent liquid.

Moreover, the flow control element 28 arranged on the purge tube 27might consist of a pump. In the device in FIG. 2, this pump might rotatecontinuously so that the impurities trapped by the membrane are removedcontinuously as they collect.

We claim:
 1. A method for cleaning a filter having a first chamber and asecond chamber separated by a filtering membrane, the first chamberhaving an outlet and having an inlet connected to a treatment liquidsource, and the second chamber having an outlet for the filteredtreatment liquid, the method comprising the steps of:conveying treatmentliquid from the source through the first chamber; recirculating during arecirculation mode, all treatment liquid from the outlet of the firstchamber to the inlet of the first chamber so that all treatment liquidexiting the first chamber is fed-back through a feedback line into thefirst chamber; adding to the recirculating treatment liquid additionaltreatment liquid from the source; measuring a clogging level of thefilter; comparing the measured clogging level with a predeterminedclogging threshold; intermittently purging during a purge mode, thefirst chamber when the measured clogging level reaches the predeterminedclogging threshold; measuring a frequency of purges of the firstchamber; comparing the measured purge frequency with a purge frequencyreference value; and increasing flow speed of treatment liquid throughthe first compartment when the measured purge frequency exceeds thepurge frequency reference value.
 2. The method of claim 1, furtherincluding the step of intermittently purging the feedback line duringthe step of intermittently purging the first chamber.
 3. The methodaccording to claim 1, wherein the step of measuring a clogging level ofthe filter comprises the substeps of:measuring a transmembrane pressureof the filter; and comparing the measured transmembrane pressure with areference transmembrane pressure.
 4. The method according to claim 3,wherein the reference transmembrane pressure is measured each time thefilter is set in operation.
 5. The method according to claim 1, whereinduring the step of recirculating a substantially constant pressure ismaintained in the second chamber of the filter.
 6. The method accordingto claim 1, wherein the step of measuring a clogging level of the filtercomprises the substeps of:measuring the speed of rotation of a pump usedfor recirculating the treatment liquid through the first chamber of thefilter; comparing the measured speed of rotation with a reference speedof rotation.
 7. The method according to claim 6, wherein the referencespeed of rotation is a speed measured each time the filter is set inoperation.
 8. A method for cleaning a filter having a first chamber anda second chamber separated by filtering membrane, the first chamberhaving an outlet and having an inlet connected to a treatment liquidsource, and the second chamber having an outlet for the filteredtreatment liquid, the method comprising the steps of:conveying treatmentliquid from the source through the first chamber; monitoring treatmentliquid flow upstream of the filter using a first flow monitoringelement; monitoring treatment liquid flow downstream of a permeatefilter using a second flow monitoring element; recirculating during arecirculation mode, all treatment liquid form the outlet of the firstchamber to the inlet of the first chamber so that all treatment liquidexiting the first chamber is fed-back into the first chamber; adding tothe recirculating treatment liquid additional treatment liquid from thesource; measuring a clogging level of the filter; comparing the measuredclogging level with a predetermined clogging threshold; intermittentlypurging during a purge mode, the first chamber when the measuredclogging level reaches the predetermined clogging threshold; andcalibrating the first and second flow monitoring elements during the steof purging by bypassing the filter, and flow connecting the first flowmonitoring element to the second flow monitoring element.